Experimental comparison of active seismic surface wave tests on shallow and deep bedrock sites

Abstract

Active seismic surface wave tests are seismic tests for which the signal is generated by artificial sources as opposed to passive tests where the signal is generated by natural source. Active tests are widely used around the world due to among other factors their non- intrusive nature. However, the use of these tests in different parts of the world is based on availability and experience. Practitioners do not always consider the technical abilities of a test relative to others, but they rather select the test based solely on the availability of that tests in that specific region. Thus, few unbiased technical comparisons of these tests have been done to date. This study investigated the comparison of three active seismic surface wave tests, spectral analysis of surface waves (SASW) test, multi-channel analysis of surface waves (MASW) test, and the continuous surface wave (CSW) test. The objective was to compare these three tests in terms of repeatability (reliability), susceptibility to near field effects, and maximum and minimum investigation depth. The tests were carried out in the field on shallow and deep bedrock sites. Midpoint SASW configuration was used with source offsets ranging from 1 m, 2 m, 4 m, 8 m, 16 m and 32 m. As there is currently no standard configuration for conducting MASW tests, MASW tests were performed using two different array configurations of 24 geophones spaced at 1 m and 2 m respectively. The source offsets used for the 1 m geophone spacing were as follows: 1 m, 2 m, 4 m, 8 m and 16 m and for the 2 m geophone spacing the offsets were set up as follows: 2 m, 4 m, 8 m, 16 m, and 32 m. CSW tests were performed using five geophones spaced at 1 m and the source offsets used were 1 m, 2 m and 4 m. 2.2 kg, 6.3 kg and 9.2 kg sledgehammers were used as energy sources for the SASW and MASW tests whereas high and low frequency shakers were used as energy sources for the CSW tests. The geophones used to detect the signal of propagating Raleigh waves were 4.5 Hz natural frequency and a PASI GEA 24 (24 bit) seismograph was used to log the signals. After the evaluation and analysis of the shallow bedrock results, all the tests were found to be repeatable (reliable) though the repeatability of CSW was better than that of SASW and MASW. MASW was found to be more prone to near field effects with the configuration of 1 m geophone spacing being affected more than the 2 m geophones spacing configuration. SASW was the second most affected and hence CSW was the least affected. Furthermore, SASW was able to sample deeper and shallower than both MASW and CSW. MASW was second best for sampling deeper and CSW was the second best for sampling shallower. At the deep bedrock site, the repeatability of the tests was better than on the shallow bedrock site however, the repeatability of MASW and CSW was better than that of SASW. With regards to near field effects, MASW test with 1 m geophones spacing configuration was affected the most, followed by SASW, then CSW. CSW was also affected by multiple modes (far field) effect. MASW test with 2 m geophones spacing configuration was affected the least. Lastly, SASW was able to sample deeper, followed by MASW and then CSW. However, SASW and MASW were able to sample to virtually the same minimum depth which was in turn shallower than that achieved by the CSW test.